The recommended way to do this is create a group which contains all your users and then assign that group to the task. However, if you really want to do this, then set the trigger to fire before the case is created in the process properties.

By the way, your question is not clear. Are you talking about adding users to the assignment list of a task or are you talking about assigning users to a task in a particular case where the task has a parallel marker?

I want all users to be able to start a process/case. For example, all users can request leave, so all of them should be assigned to it. But I wouldn't like to be adding the new users into that group every time, so is there anyway to do this dynamically? With a cron job perhaps?